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The Solvable Problem of Traffic Congestion

Note the word “solvable” in the title. For some unknown reason, otherwise intelligent people talk as if urban traffic congestion is a fact of life that we’ll never solve forever amen.

I disagree. The problem is solvable if we are willing to make tougher decisions than we’ve made so far.

First we have to have a quick understanding of traffic congestion: the main problem is that you have more cars entering the road than you have leaving the road. If you have equal or less cars entering the road as leaving the road, you have static or decreasing congestion. When more cars are entering the road system than leaving it, trip times increase, and when trip times increase, cars get off the road slower yet, and traffic gets worse exponentially.

It’s important to understand that it’s not just the width of the road, but with what efficiency it is being used. When you think of cars think of trip times and when you think of roads thinks of cars/second. These are the dominant factors that cause inflationary congestion. Trip times get longer because roads move cars slower.

Because there are so many cars on the road, small efficiencies make a big difference. If we increase trip times by 40% and the average trip time is 20 minutes, that’s 8 more minutes per car. Take that times 50,000 cars and you have 6000 more car hours to deal with than you would have. It pays to stop congestion before it starts.

So we have two areas we need to make sure we keep efficient: trip times and cars/sec, which are clearly related.

The answer is flow. Traffic is like a data network. It should go as fast as it can and it should be slowed down never. Our freeways are currently clumpy, chaotic battlegrounds where people jockey for position. If you think of driver behavior as a bell curve, with the slowest drivers on one end and the fastest drivers on the other, the peak of the bell curve is pretty wide — these are the rational drivers. We need to rid the road of the 20% worst drivers (10% on either side). It should be harder to get and keep your driver’s license. The problem is, we are enforcing the wrong side. It is not just the fastest drivers that should be pulled over, but the slowest and the stupidest. All drivers who drive non-cooperatively should be off the road. We have to stop enforcing speed and start enforcing cooperative driving behavior.

Except in residential areas, you have to pretty much throw out speed limits. They aren’t working. The middle of the bell curve above, if speed limits were working, would be centered on the speed limit. Instead it is centered 10 mph (or more) above the speed limit. At the edge of the bell curve you have a small minority of people who obey the speed limit, and they become dangerous eddies in the traffic flow and end up causing congestion and accidents. You always hear about how “speeds kills”. This is only true if you have accidents. It’s the accidents that do the killing and accidents are much more likely when you have large differences in speed. With 95% of the people driving 10 or 20 miles per hour over the speed limit, the 5% going the speed limit are dangerous obstacles. You can’t argue with the middle of the bell curve.

So this is one way to smooth out the traffic flow: enforce a very small range in speeds. Perhaps the freeways have a speed limit of 72-74. Or perhaps the speed limit signs change to regulate speed. If you truly enforced speed, but had speed limits where they should be, you could control traffic flow, perhaps even dynamically.

Another traffic control option would be to restrict lane changes during capacity traffic. This creates the oscillating speeding up and slowing down. When you merge traffic in during capacity traffic you need to do it slowly, even though it will piss off the people trying to get on the road. Road systems need to be designed to move people as fast as possible. Traffic control systems need to be intelligent, with a model of the traffic dynamic, and keeps cars and traffic moving. Our traffic control systems are way too stupid. We have a system that stops you every 1 block. We put traffic lights on roads that should be throughways. The only tool we seem to have in our toolbox is slowing and stopping traffic, yet that is the main culprit in creating congestion. We can’t just build lanes, we need to build dynamic traffic control systems.

So in a nutshell, to solve traffic congestion on an on-going basis:

1. Raise the standards for getting and keeping your license. Let’s get non-cooperative drivers off the road.
2. Raise speed limits to the speed that people are actually driving and then enforce the hell out of it.
3. Stop or slow down traffic as seldom as possible, hopefully never. This is a major change from current systems.
4. Educate drivers and then enforce cooperative, attentive and rational driving.
5. Expand and modernize infrastructure. Modern cities need modern road systems.